In Drosophila embryos the nuclei divide with global synchrony during the first 13 mitotic cycles. This synchrony is lost in the 14-th cycle. Starting about one hour into the 14-th interphase, at least 25 'mitotic domains', which axe clusters of cells united by locally synchronous mitosis, subdivide the embryo blastoderm surface into a complex spatial pattern. This pattern, which partitions the blastoderm more finely than current fate maps, is reproducible, bilaterally symmetric, and precisely aligned with respect to the expression of the engrailed gene. Cells in at least some mitotic domains share specific morphogenetic traits, distinct from those of cells in adjacent domains. These traits include cell shape, spindle orientation, and participation by all the cells of a domain in an invagination. The specialized behaviors of the various mitotic domains transform the monolayer cell sheet of the blastoderm into the multi-layered gastrula. The fine-scale subdivision of the newly cellularized embryo into mitotic domains appears to be a manifestation of the commitment of cells to specific developmental fates. Studies of the fate, action, morphogenetic autonomy, and lineage restraints of cells in individual domains will be performed in order to test this hypothesis. As a spin-off, these studies win refine current blastoderm fate maps aligning them with the mitotic domain atlas. Embryos from other species will be examined for the presence of mitotic domains; the goal is to lay the foundation for a bridge connecting the new molecular-level knowledge of fly development to the developmental biology of organisms closer to the "human branch" of the phylogenetic tree.